(a) Field of the Invention
The present invention concerns a power amplifier designed to enhance power efficiency by switching power supply voltages in accordance with the magnitude and the frequency of the input signal to be amplified.
(b) Description of the prior art
In the past, known power amplifiers existed which were intended, for example, for audio systems, and arranged so that the power supply voltage which is supplied to an output amplifying device was varied between high level and low level in accordance with the amplitude of the input signal to be amplified, to thereby reduce the power loss of the output amplifying device and to improve power efficiency. Power amplifiers of this type may be divided into two groups. The first is arranged such that, as shown by way of the waveshape in, for example, FIG. 1, in the region wherein the output voltage e.sub.0 exceeds a predetermined level, the power supply voltage +V.sub.S (or -V.sub.S) undergoes a stepwise increase or switching from a voltage level +V.sub.L (or -V.sub.L) to a voltage level +V.sub.H (or -V.sub.H). The second group is arranged such that, as shown in FIG. 2, in the region wherein the output voltage e.sub.0 exceeds a predetermined level, the power supply voltage +V.sub.S (or -V.sub.S) is varied bootstrapwise from the voltage +V.sub.L (or -V.sub.L) only for the amount corresponding to the change in the voltage e.sub.0. In each of these two types of known power amplifiers, arrangement is made so that the switching-over of power supplies is carried out with a certain delay time to prevent the occurrence of problems such as, in the case where the frequency of the input signal is high, the power supplies are frequently switched over between them, causing noise generation, and developing distortions. Accordingly, in case the frequency of the input signal is low, as shown in the period T.sub.1 in FIGS. 1 and 2, the connection of the power supplies is switched over in real time substantially corresponding to the region in which the output voltage e.sub.0 exceeds a predetermined level. However, in case the frequency of the input signal is high as shown in the period T.sub.2 in FIGS. 1 and 2, the power supply voltage +V.sub.S (or -V.sub.S) will be held in its elevated state even when the output voltage e.sub.0 stays in the region below the predetermined level. Therefore, in the conventional power amplifier of this type, there has been the problem that in case the frequency of the input signal is high, the effect of improvement of power efficiency becomes lowered.
Next, consideration will be made of the voltage which is applied to an output amplifying device in the conventional power amplifier of this type, with respect to the instance wherein the power amplifier is constructed with a push-pull amplifying circuit using an NPN transistor 1 and a PNP transistor 2 to serve as the output amplifying devices as shown in FIG. 3.
In this circuit, the voltages V.sub.C1 and V.sub.C2 which are applied across the collectors and the emitters, respectively, of the transistors 1 and 2 become a voltage (V.sub.L +V.sub.H) which is maximum at, for example, time t.sub.2 and t.sub.1 for an input signal having a low frequency as that shown in the period T.sub.1 in FIGS. 1 and 2. For an input signal having a high frequency as shown in the period T.sub.2, the voltages V.sub.C1 and V.sub.C2 will become 2V.sub.H at maximum, respectively, which is of a value greater than that (V.sub.L +V.sub.H) for the signal of low frequency. Thus, in the conventional power amplifier, it will be noted that, when the frequency of the input signal becomes high, the voltage which is applied to the output amplifying device will become elevated as compared with the instance wherein the frequency of the input signal is low. Hence, there has been the problem that the maximum rated voltage, especially V.sub.CEO, of the output amplifying device has to be selected higher correspondingly for the amount of the elevation. This has been a very important problem especially in case the power amplifier of this type is intended to be applied to the power amplification for a large output.